Ash Sutton dominated round 26 to strengthen his championship lead while Tom Ingram dropped out of the title race despite another podium finish.
Conditions were markedly different with a drying track and a split of drivers on wet and dry tyres.
Sutton got a better start than polesitter Dan Cammish, taking the lead before turn 1.
Jake Hill, Rory Butcher and Ollie Jackson connected at turn 1 putting Hill and Jackson into the gravel while in a separate incident Cammish and Ingram made contact putting the Toyota driver in the gravel and outside the top 10.
It was the wet-shod drivers who were immediately quicker with Tom Chilton taking the lead and putting a good gap between himself and second placed Sutton.
On a drying track, it was unsurprising that Sutton closed the gap to Chilton and was finally able take the lead from Chilton at Surtees on lap 9.
Behind the leaders Josh Cook led a train of cars including Cammish, Ingram and Turkington.
The battle came to a head on lap 12 with Cammish lunging on Cook at Paddock Hill Bend but both drivers went wide opening the door for Ingram to overtake them both.
The battle carried on into Druids where Turkington and Cook made contact leaving the BMW with slight damage at the right front of the car.
Despite the damage Turkington was able to catch third-placed Cammish but was unable to pass the Honda driver.
But on lap 22, Paul Rivett spun on the exit of Paddock Hill Bend and was stuck in the middle of the track.
The leaders came through at full speed with Turkington overtaking Cammish as they avoided the stricken Audi.
From there Turkington hung on to the final podium position ahead of Cammish and Ollie Jackson.
Sutton goes into the final round with a gap of 9 points to Turkington with Cammish a further 13 points behind.
Adam Morgan, Jake Hill and Senna Proctor finished in sixth, seventh and eighth respectively with Josh Cook and Rory Butcher rounding out the top 10.
Chris Smiley came home 11th ahead of Aidan Moffat.
13th place was enough to give Michael Crees the 2020 Jack Sears Trophy ahead of 2nd placed Carl Boardley.
Sam Osbourne finished 14th with Tom Oliphant rounding out the top 15.
Round 26 Results
1 – Ash Sutton 21:14.892s
2 – Tom Ingram +4.762s
3 – Colin Turkington +7.688s
4 – Dan Cammish +9.370s
5 – Ollie Jackson +10.832s
6 – Adam Morgan +13.422s
7 – Jake Hill +14.088s
8 – Senna Proctor +14.418s
9 – Josh Cook +14.919s
10 – Rory Butcher +17.900s
11 – Chris Smiley +22.605s
12 – Aidan Moffat +28.823s
13 – Michael Crees +33.011s
14 – Sam Osbourne +37.119s
15 – Tom Oliphant +37.712s
